DBCC SHRINKFILE (Transact-SQL)

Shrinks the size of the specified data or log file for the current database or empties a file by moving the data from the specified file to other files in the same filegroup, allowing the file to be removed from the database. You can shrink a file to a size that is less than the size specified when it was created. This resets the minimum file size to the new value.

Is the identification (ID) number of the file to be shrunk. To obtain a file ID, use the FILE_IDEX system function or query the sys.database_files catalog view in the current database.

target_size

Is the size for the file in megabytes, expressed as an integer. If not specified, DBCC SHRINKFILE reduces the size to the default file size. The default size is the size specified when the file was created, or the last size set by using ALTER DATABASE.

If target_size is specified, DBCC SHRINKFILE tries to shrink the file to the specified size. Used pages in the part of the file to be freed are relocated to available free space in the part of the file retained. For example, if there is a 10-MB data file, a DBCC SHRINKFILE operations with a target_size of 8 causes all used pages in the last 2 MB of the file to be reallocated into any unallocated pages in the first 8 MB of the file. DBCC SHRINKFILE does not shrink a file past the size needed to store the data in the file. For example, if 7 MB of a 10-MB data file is used, a DBCC SHRINKFILE statement with a target_size of 6 shrinks the file to only 7 MB, not 6 MB.

EMPTYFILE

Migrates all data from the specified file to other files in the same filegroup. Because the Database Engine no longer allows data to be placed in the empty file, the file can be removed by using the ALTER DATABASE statement.

NOTRUNCATE

Moves allocated pages from the end of a data file to unallocated pages in the front of the file with or without specifying target_percent. The free space at the end of the file is not returned to the operating system, and the physical size of the file does not change. Therefore, when NOTRUNCATE is specified, the file appears not to shrink.

NOTRUNCATE is applicable only to data files. The log files are not affected.

TRUNCATEONLY

Releases all free space at the end of the file to the operating system but does not perform any page movement inside the file. The data file is shrunk only to the last allocated extent.

DBCC SHRINKFILE applies to the files in the current database. For more information about how to change the current database, see USE (Transact-SQL)

DBCC SHRINKFILE operations can be stopped at any point in the process, and any completed work is retained.

Note:

In SQL Server 2005, when a DBCC SHRINKFILE operation fails an error is raised. In earlier versions of SQL Server, this operation failed without raising an error.

The database being shrunk does not have to be in single-user mode; other users can be working in the database when the file is shrunk. You do not have to run the instance of SQL Server 2005 in single-user mode to shrink the system databases.

Shrinking a Log File

For log files, the SQL Server 2005 Database Engine uses target_size to calculate the target size for the whole log; therefore, target_size is the amount of free space in the log after the shrink operation. Target size for the whole log is then translated to target size for each log file. DBCC SHRINKFILE tries to shrink each physical log file to its target size immediately. However, if part of the logical log resides in the virtual logs beyond the target size, the Database Engine frees as much space as possible, and then issues an informational message. The message describes what actions are required to move the logical log out of the virtual logs at the end of the file. After the actions are performed, DBCC SHRINKFILE can be used to free the remaining space. For more information, see Shrinking the Transaction Log.

Because a log file can only be shrunk to a virtual log file boundary, shrinking a log file to a size smaller than the size of a virtual log file might not be possible, even if it is not being used. The size of the virtual log file is chosen dynamically by the Database Engine when log files are created or extended. For more information about virtual log files, see Transaction Log Physical Architecture.

Best Practices

Consider the following information when you plan to shrink a file:

A shrink operation is most effective after an operation that creates lots of unused space, such as a truncate table or a drop table operation.

Most databases require some free space to be available for regular day-to-day operations. If you shrink a database repeatedly and notice that the database size grows again, this indicates that the space that was shrunk is required for regular operations. In these cases, repeatedly shrinking the database is a wasted operation.

A shrink operation does not preserve the fragmentation state of indexes in the database, and generally increases fragmentation to a degree. This is another reason not to repeatedly shrink the database.

Troubleshooting

This section describes how to diagnose and correct issues that can occur when running the DBCC SHRINKFILE command.

The File Does Not Shrink

If the shrink operation runs without error, but the file does not appear to have changed in size, verify that the file has adequate free space to remove by performing one of the following operations:

SELECT name ,size/128.0 - CAST(FILEPROPERTY(name, 'SpaceUsed') AS int)/128.0 AS AvailableSpaceInMB
FROM sys.database_files;

Run the DBCC SQLPERF command to return the space used in the transaction log.

If insufficient free space is available, the shrink operation cannot reduce the file size any further.

Typically it is the log file that appears not to shrink. This is usually the result of a log file that has not be truncated. You can truncate the log by setting the database recovery model to SIMPLE, or by backing up the log and then running the DBCC SHRINKFILE operation again. For more information, see Transaction Log Truncation and Shrinking the Transaction Log.

The Shrink Operation Is Blocked

It is possible for shrink operations to be blocked by a transaction that is running under a row versioning-based isolation level. For example, if a large delete operation running under a row versioning-based isolation level is in progress when a DBCC SHRINK DATABASE operation is executed, the shrink operation will wait for the delete operation to complete before shrinking the files. When this happens, DBCC SHRINKFILE and DBCC SHRINKDATABASE operations print out an informational message (5202 for SHRINKDATABASE and 5203 for SHRINKFILE) to the SQL Server error log every five minutes in the first hour and then every hour after that. For example, if the error log contains the following error message:

DBCC SHRINKFILE for file ID 1 is waiting for the snapshot
transaction with timestamp 15 and other snapshot transactions linked to
timestamp 15 or with timestamps older than 109 to finish.

This means that the shrink operation is blocked by snapshot transactions that have timestamps older than 109, which is the last transaction that the shrink operation completed. It also indicates that the transaction_sequence_num, or first_snapshot_sequence_num columns in the sys.dm_tran_active_snapshot_database_transactions dynamic management view contains a value of 15. If either the transaction_sequence_num, or first_snapshot_sequence_num columns in the view contains a number that is less than the last transaction completed by a shrink operation (109), the shrink operation will wait for those transactions to finish.

To resolve the problem, you can do one of the following tasks:

Terminate the transaction that is blocking the shrink operation.

Terminate the shrink operation. If the shrink operation is terminated, any completed work is retained.

Do nothing and allow the shrink operation to wait until the blocking transaction completes.

A. Shrinking a data file to a specified target size

B. Shrinking a log file to a specified target size

The following example shrinks the log file in the AdventureWorks database to 1 MB. To allow the DBCC SHRINKFILE command to shrink the file, the file is first truncated by setting the database recovery model to SIMPLE.

D. Emptying a file

The following example demonstrates the procedure for emptying a file so that it can be removed from the database. For the purposes of this example, a data file is first created and it is assumed that the file contains data.